Control systems that are condition responsive are well known. One of the major applications of this type of condition responsive control system is in the control of heating and cooling equipment. The present invention is generally applicable to any type of condition control system that utilizes a condition responsive control, but will be generally described in terms of a thermostatically controlled system or thermostat.
A thermostat typically uses thermal anticipation to obtain a better system performance. This anticipation reduces the dependence on the ambient space temperature to actuate the thermostat between its "on" and "off" conditions. Various means are used to obtain the anticipation heat, but all of these are thermal and are, therefore, subject to the different air flows that exist in different installations. If the actual air flow over the thermostat in a particular application is greater or less than the air flow the thermostat was designed for, the actual temperature rise of the sensor due to the anticipator will be reduced or enhanced. This will result in less than optimum performance. A similar effect will occur if the air flow changes from time to time in a given installation. If the air flow is constant, the anticipator can be readjusted to bring back optimum performance, but in changing air flow conditions no one setting will be optimum. It should also be noted that in most thermostats, a change in the characteristics of the anticipator will also change the entire system droop.
In an electronic thermostat, anticipation can be achieved electronically. This has the advantage of not being affected by air flow and thus eliminates all of the problems associated with thermal anticipation as noted above. One method of obtaining this type of anticipation is the use of a resistor and capacitor charge and discharge arrangement as part of the negative feedback of an electronic amplifier while using a fixed positive feedback. This type of electronic anticipation is injected as a negative feedback mode with a single order time constant. For proper system operation, this time constant may need to be in the order of sixteen minutes. To obtain this type of a time constant with a single resistor-capacitor arrangement requires high resistances and a very low leakage, large capacitor. The size of the resistors and capacitor would place a burden on the cost of the device, and on the physical size of the thermostat itself, making electronic anticipation obtained in this fashion impractical for many thermostatic applications.
In the U.S. Pat. No. 4,196,356 to Kabat and the U.S. Pat. No. 4,186,315 to Benton, a prior art condition responsive time proportional control means has been specifically disclosed. The time proportional circuit utilizes a relatively small capacitor and resistors having a rapid cycling rate. This rapid cycle controls a counter that forms part of a counting means. The counter, in one simple form, is a ripple counter. The cycling action of the time proportional control means is combined with a pulse generating means so that the time constant of the overall control system can be multiplied by the pulse rate of the pulse generating means without changing the system droop (the temperature cycling band).
In the previously mentioned prior art types of condition responsive time proportional control means, a problem has arisen in the application of the control system under certain operating conditions. It has been found in the prior art devices that the integrating action of the counter can cause an undesirable cycling of the load. Under load conditions of approximately ten to ninety percent of full load for the system, the prior art devices work quite well. In the very light load conditions and the very heavy load conditions, the counter integrating action can disrupt the operation of the system when normal cycling room temperature swings go outside of the proportional band. This disruption occurs in that a longer than normal time delay is required to load the counter once the room temperature swings back into the proportional band, and the overall system performance is less than desirable. This action has been referred to as a "gulping" action. This was corrected in the Benton patent by providing the counting means with two separate counting channels. The correction of this action has been referred as a "degulping" of the system.